The siege of Limonum, early 51 B.C., was an unsuccessful attempt by the Andes, one of the last rebellious tribes in Gaul, to capture the chief town of the Pictones tribe. After the destruction of Vercingetorix's army at Alesia the remaining Gallic rebels decided not to raise another central army and instead to try and wear the Romans out with constant uprisings around the edges of the country.
One of those uprising was led by Dumnacus, the general of the Andes tribe, which occupied the area to the north of the lower Loire. While Caesar and most of his legions were occupied in the north-east, putting down the Bituriges, Carnutes and Bellovaci tribes, Dumnacus led his army south of the Loire, into the lands of the Pictones, and laid siege to Limonum (modern Poitiers).
The nearest Roman troops were two legions under Caius Caninius Rebilus. At the start of the winter of 52-51 B.C. he had been posted in the lands of the Ruteni, close to the north-western border of the Roman Province in southern Gaul, but by the late winter he had been given authority over a large part of western Gaul. Caesar was clearly worried that Caninius didn't have enough men to hold down such a large area, and so after the defeat of the Bellovaci he sent Caius Fabius and twenty five cohorts (two and a half legions) west to reinforce him.
Fabius had not yet arrived in the area when letters reached Caninius from Duracius, one of Rome's allies in the west, informing him of the situation in the country of the Pictones. Caninius led his legions towards Limonum, but as he approached the town he discovered that the town was being besieged by at least 12,000 men, and that Caninius was trapped inside. Aware that his legions were not particularly strong, Caninius built a well sited camp, and wrote to Fabius to inform him of the situation.
Dumnacus responded to the arrival of the Romans close to Lumonum by attacking their camp, but after several days of futile and costly assaults he turned back to continue his attack on the town.
The siege was only raised when Dumnacus discovered that Fabius, with his two and a half legions, was approaching. Dumnacus realised that he was about to be trapped between a strong Roman army and the defenders of the town, and decided to retreat back across the Loire. The Gauls were intercepted by Fabius before they could reach safety, and the retreating army suffered a very heavy defeat (battle on the Loire).